Reinforcing fiber composite materials have attracted attentions since it is light and has characteristics of excellent heat resistance and mechanical strength, and thus, they have been more widely used in applications of various structures such as casings or various members of automobiles and aircrafts. Matrix resins for the fiber reinforced composite materials are required to have various types of performance, such as superior impregnation into reinforcing fibers, excellent storage stability, no generation of voids or the like even with high curability, and excellent heat resistance, mechanical strength, and fracture toughness in terms of the cured product. There has been a demand for development of a resin material having a good balance among these various types of performance.
As an epoxy resin having high curability and exhibiting excellent heat resistance in terms of a cured product thereof, bisphenol A novolac type epoxy resins have been known. In particular, among the bisphenol A novolac type epoxy resins, those capable of improving the heat resistance for a cured product by attaining higher molecular weights of the resins have been proposed (see, for example, PTL 1).
On the other hand, a bisphenol A novolac type epoxy resin having a higher molecular weight easily generates 3-dimensional bridges within the molecular structure, and has a reduced reactivity of an epoxy group, and as a result, the change in heat resistance after the thermal history in cured products thereof easily occurs, and in addition, the thermal linear expansion properties are also deteriorated. In view of these, through the modification of the resins, a curable resin composition formed by using a polyglycidyl ether of bisphenol novolac having a ratio of the bindings of the 2-, 4-, and 6-positions in a phenol nucleus set to a specific range has been proposed (see, for example, PTL 2).
However, such a curable composition has excellent curability, while the number of epoxy groups decreases in an intramolecular reaction, and as a result, a cured product thereof has insufficient mechanical strength, and thus, has low toughness, so-called fracture toughness.